The present invention relates to electronic ballasts for high pressure discharge lamps, such as high pressure mercury lamps and metal halide lamps, an illumination fixture using lamps powered by such a ballast, and a light source for a projector.
FIG. 1 is a circuit diagram showing a basic configuration for a high pressure discharge lamp ballast. A voltage supplied from a DC power source 1 is stepped down by a buck converter 2 whose output is converted into a rectangular wave AC voltage by an inverter (polarity inversion) circuit 3. A high voltage is generated for lamp ignition by a resonant circuit 4 which is connected to an output of the inverter circuit 3 and includes a capacitor C2 and an inductor L2. A voltage detection circuit 5 detects a voltage applied to a high pressure discharge lamp La.
To ignite a high voltage discharge lamp using such a high pressure discharge lamp ballast, pairs of switching elements Q2 and Q5 and switching elements Q3 and Q4 arranged in respective positions diagonal from each other in the inverter circuit 3 are turned on/off alternately at a high frequency to generate a high frequency voltage in a range from several tens kHz to several hundreds kHz across the resonant circuit 4. This high frequency voltage is boosted by a resonance action of the resonant circuit 4, sweeping a driving frequency in the switching elements Q2 to Q5 until the capacitor C2 is allowed to reach a desired voltage, followed by fixing a driving frequency to maintain and generate a high voltage when a desired high pressure resonance voltage is obtained. This high pressure resonance voltage is then used to cause a breakdown of the high pressure discharge lamp La.
As stated above, to obtain a high resonance voltage for causing a breakdown of the high pressure discharge lamp in the lamp ballast of a resonant-start type, in which a high frequency voltage is applied by the resonant circuit 4 at the time of starting, the inverter circuit 3 is subjected to switching by a resonance frequency (or frequency obtained by multiplying a resonance frequency by one over an odd number) applied to the inductor L2 and the capacitor C2 in the resonant circuit 4. The resonance action is used to generate a starting voltage for the high pressure discharge lamp for a certain period of time. This is called a starting (or ignition) mode Tst.
Thereafter, to promptly preheat one or more lamp filaments, a frequency in the inverter circuit 3 is reduced to a driving frequency which is relatively lower than a driving frequency used for an operation in the ignition mode, whereby the current flowing into the high pressure discharge lamp is increased to preheat the lamp filaments. This is called a preheating mode Tpre.
This preheating mode Tpre is followed by application of a low-frequency rectangular wave voltage to the high pressure discharge lamp for continuous arc discharge, so that lighting of the high pressure discharge lamp is maintained. This is called a normal lighting mode and/or normal mode Tnorm.
FIG. 19 shows temporal changes made in a voltage applied to a high pressure discharge lamp and a driving frequency in an inverter circuit when no breakdown occurs during ignition mode Tst periods. FIG. 20 shows temporal changes made in a voltage applied to the high pressure discharge lamp, a current flowing in the high pressure discharge lamp and a driving frequency in the inverter circuit when a breakdown occurs in the ignition mode Tst periods. In each period of the ignition mode Tst, a voltage is applied to a high pressure discharge lamp La with a frequency which is sufficiently higher than a lighting frequency used in normal lighting, and the pairs of the switching elements Q2 and Q5 and the switching elements Q3 and Q4 of the inverter circuit 3 are subjected to an alternate switching operation at a high frequency.
However, driving the inverter circuit 3 at a relatively high frequency as observed in each period of the ignition mode will result in a high impedance in the load circuit, making it difficult to supply adequate preheat current to the high pressure discharge lamp. Therefore, in each period of the ignition-mode in which a breakdown of the high pressure discharge lamp is triggered, a period of time required to achieve breakdown of the high pressure discharge lamp varies due to individual variations in high pressure discharge lamps, and further depending on temperature and atmospheric pressure of a high pressured discharge lamp resulting from a period of time elapsed from extinguishing of the high pressure discharge lamp to the time of restarting. Therefore, to preheat the lamp filaments, the ignition-mode periods are generally followed by preheating mode periods in which the driving frequency in the inverter circuit is lower than the frequency applied in the ignition mode, thereby reducing impedance in the load circuit with an increase of a current flowing into the high pressure discharge lamp. An operation to cause a high frequency current to flow in a positive/negative symmetrical manner is  maintained for a predetermined period of time.
When the preheating mode is complete the process will move on to the normal mode, which refers to each period of the normal lighting mode wherein the pairs of the switching elements Q2 and Q5 and the switching elements Q3 and Q4 are turned on/off alternately at a low frequency to generate a low frequency voltage in a range from several tens Hz to several hundreds Hz across the high pressure discharge lamp to maintain proper lighting.
As stated above, in a control operation carried out by setting a predetermined period of time for an ignition mode to cause a breakdown of a high pressure discharge lamp upon startup, and for the preheating mode to preheat a lamp filament, transition to the preheating mode to appropriately preheat the lamp filament follows completion of the ignition mode and is therefore accompanied by a time lag which occurs in control switching, leaving concern about deterioration in the ability of the lamp to ignite.
In a second conventional example as shown in FIGS. 21 and 22, to secure starting operation even if high pressure discharge lamps having different characteristics caused by manufacturing variations, and the starting voltage is increased at an end-of-life stage thereof, starting control is carried out by alternately turning on/off the pairs of the switching elements Q2 and Q5 and the switching elements Q3 and Q4 in the inverter circuit 3, while sweeping a driving frequency in a predetermined frequency range so as to pass through a resonance point of the resonant circuit 4. Moreover, for the purpose of reducing the size of components which constitute the resonant circuit 4 while obtaining a voltage amplitude which is substantially the same as that obtained in driving the inverter circuit 3 at the above frequency, the frequency obtained by multiplying a resonance frequency by one over an odd number (i.e., 1/(2n+1), where n is a whole number) is occasionally set as a driving frequency for use in performing a starting control for the inverter circuit 4. The amplitude of a resonant voltage obtained by such a frequency is tapered as the multiplier is raised, and particularly when the multiplier is set to three times, it is possible to obtain a voltage amplitude which is substantially equivalent to that obtained in driving the inverter circuit 3 at a resonance frequency determined by the inductor L2 connected in series to the high pressure discharge lamp and the capacitor C2 connected in parallel therewith, so that component size reduction in the resonant circuit 4 can be realized.
In comparison with the control technique according to the first conventional example, the second conventional example causes large stresses to the circuit from sweeping a frequency through a resonance point in the ignition mode. However, using a driving frequency obtained by multiplying the resonance frequency in the resonant circuit by one over an odd number such as ⅓ times and ⅕ times make it possible to reduce component stresses while obtaining a substantially similar voltage amplitude.
In the case where lighting of a high pressure discharge lamp is achieved in the ignition mode, current flowing into the high pressure discharge lamp can be effectively increased in comparison with the control technique according to the first conventional example because driving control is performed at a frequency lower than a resonance point (or a point obtained by multiplying a resonance point by one over an odd number). However, even with the control technique according to the second conventional example, it is impossible to sufficiently heat lamp filaments immediately after startup and further improvement of lamp start-up is required.
In an alternative example of a lamp ballast as previously known in the art, and with reference to the basic configuration shown in FIG. 1, it has been proposed that the driving frequency in the inverter circuit 3 at startup is set to a frequency close to a frequency obtained by multiplying a resonance frequency of the resonant circuit 4 by one over an odd number.
As stated above, a high pressure discharge lamp ballast of a resonance activation type generally sets a predetermined period of time, which is about a few seconds for ignition mode periods, in which breakdown of a high pressure discharge lamp is triggered, because a period of time required to achieve breakdown varies among individual high pressure discharge lamps and depends on a temperature and an atmospheric pressure of a high pressure discharge lamp resulting from a period of time elapsed between extinguishing the lamp and restarting. Accordingly, a relatively high driving frequency observed in the periods after achieving a breakdown of the high pressure discharge lamp causes reduction of a current flowing into the high pressure discharge lamp, wherein the high pressure discharge lamp may not be effectively preheated during a period of time before transition to the preheating mode, shutting down the lamp in a worst case and occasionally causing significant stress in the high pressure discharge lamp due to repeated extinguishing and ignition.